Field of the Invention
This invention relates to an aqueous agent for the anticorrosive treatment of metallic substrates and to a method for coating such substrates.
Discussion of Related Art
Generally, metallic substrates such as sheet steels for the automotive industry are disadvantageously subject to corrosion on their surfaces.
The corrosion can, however, be counteracted by applying a coating and/or by producing a conversion layer by a conversion treatment of the relevant substrate. In the following, the conversion layer is understood to be a layer that is produced through chemical transformation (conversion) of the substrate on its surface and of various components of an aqueous passivation agent.
A conversion treatment is often used as a pretreatment. In such cases, the pretreatment is used on the metal surface in order to improve its corrosion resistance and also the adhesion properties. In this way, the surface is prepared to be provided with another organic coating.
German Patent Reference DE 10 2006 000 600 B4 discloses a method for coating metallic surfaces. The aqueous composition used for the coating includes at least one phosphate, at least one zirconium compound, a complexing agent, and cations of aluminum and/or zinc.
Due to negative environmental influences, however, methods that use phosphate compounds and chromium compounds are increasingly being replaced with alternative methods.
One possible alternative is the use of acidic aqueous solutions of fluoro complexes which have known anticorrosive properties.
German Patent Reference DE 10 2008 014 465 A1, for example, discloses an aqueous, chromium-free agent for anticorrosive conversion treatment of metallic surfaces. The agent contains zirconium- and fluorine compounds as well as water-soluble compounds that release iron- and copper ions. After the conversion treatment, the metallic surface undergoes a subsequent dip painting.
If the conversion treatment is to serve as a pretreatment for an additional coating, then stricter requirements are placed on the adhesion properties of the conversion layer. Both the adhesion between the substrate and the conversion layer and the adhesion between the conversion layer and the additional layer must be improved in order to avoid negative effects such as the infiltration of rust underneath the anticorrosive layers as much as possible.
EPO Patent Reference EP 1 900 846 A1 describes a method for the chemical conversion treatment of metallic substrates. According to the method in EPO Patent Reference EP 1 900 846 A1, zirconium and fluorine serve as components for producing the conversion layer and as agents for etching the metal surface so that it is possible to increase the corrosion resistance. In addition, an alkoxysilane, which has one amino group, is used as another component for improving the adhesion of the conversion layer both to the substrate and to a coating that is to be applied subsequently.
When using silanes that have functional groups in the form of amino groups, however, it is important to note that the adhesion of the conversion layer to a paint layer that is applied over it does not always meet the imposed requirements.
There remains a need for agents and methods for anticorrosive treatment of metallic substrates, which in addition to anticorrosive properties, also have optimal adhesion properties, thus making it possible to largely avoid the danger of infiltration under the paint.